The present invention relates to arrangements for radio communication comprising first station(s) or base station(s), a number of second radio stations and a number of subscriber stations wherein said radio station(s) comprise(s) at least one digital radio arrangement.
Up to now the connections to the overwhelming majority public telephone subscribers, also to residences, offices, is made by wiring. This means that even if the network implements radio technology, e.g. by satellite, the last part of the network mostly uses wiring, i.e. most local connections are in the form of wires. However, the installation of wired connections especially for the local loop e.g. residences, offices, etc is both time consuming and involves substantial networking costs. Therefore there is an increasing interest in exploring the potential of radio technology as an alternative for wiring in the local loop. This is among others of general interest for new second and third public network operators since it provides fast installation and can provide local mobility as an advantage. For a first dominant operator it is of interest at least for niche markets. Another problem, however not general but merely relevant to some countries, is that the regulations in these particular countries for public telephone operators prevent them from offering mobility in the PSTN/ISDN (Public Switched Telephone Network/Integrated Services Digital Network).
Some attempts have been done to use radio link connections to the subscriber. The concept itself is called Radio in the Local Loop, or in short form RLL. The traditional, wired, connection, will in the following be called Wired Local Loop, WLL. FIG. 1 show a simplified illustration of Wired Local Loops, WLL- and Radio Local Loops, RLL-connections with public subscribers, residences as well as offices. BS Stands for Base Station.
As to the concept of the Radio in the Local Loop, RLL, this can be divided into two basic concepts, namely the so called Fixed Radio in the Local Loop, FRLL, and the Mobile Radio in the Local Loop, MRLL. These two concepts are illustrated in FIGS. 2 and 3 respectively.
In FIG. 2 the Fixed Radio in the Local Loop concept is illustrated. In this case the subscriber has one or more telephones sockets 6' to which his telephone or telephones 4' are connected. In this case there is not really a difference for the subscriber from having a Wired Local Loop since he does not see any difference. The telephones 4' are connected to the telephone sockets 6' which are connected to a so called Subscriber Fixed Station, SFS, 2', which is connected to an antenna 7', e.g. arranged on the roof of a building or similar via which is established a radio connection with abase Station, BS 1', 1". The short dashed arrow indicates that there is little interference between adjacent base stations 1', 1". Up to now the Fixed Radio in the Local Loop, FRLL, concept has only been implemented to a very limited extent. Radio microwave link connections or special developed radio technology is merely used to provide telephone service for isolated islands, remote farms or similar. Lately however, FRLL-systems based on existing analogue cellular technology have been installed in countries which have a general lack of capacity in the public wired telephone network. One implementation is further described in brochure no. LZT 120217, Radio ACCESS System RAS 1000 by Ericsson Radio Access AB. These systems are addressing a time limited market, since general applications will require effective encryption of a high quality voice and ISDN (Integrated Services Digital Network)-service, in order to achieve that a digital radio access technology is required. An essential feature of the Fixed Radio in the Local Loop, FRLL, is that it enables installations which are economic as to frequency and power since directional fixed roof mounted antennas can be installed at the subscriber buildings. For example a 15 dB antenna gain could give 20-30 dB back-front isolation leading to a re-use cluster size as low as 1 leading to typically ten times higher frequency. effiency compared to the Mobile Radio in the Local Loop, MRLL, which is going to be further discussed hereinafter. This frequency efficiency is essential for providing the higher bit rates required for the high voice quality as well as for ISDN-services. The directional antennas also reduce the potentional risk for quality degradation due to time dispersion. An important drawback with the FRLL is however that the customer does not get any mobility benefit.
Mobility is provided through the MRLL, which is an advantage. This concept however also suffers from drawbacks, e.g. the radio infra-structure becomes very frequency inefficient and requires very expensive base station installations due to range limitations. The reason therefore is that a perfect radio connection is required in every part of the house independent of building materials, underground basements, topography, temporary placement or position of the portable telephone etc. This is very hard and expensive to achieve. Furthermore, no antenna gain will be available on the customer's side and the mobile units can not be high power units. This could easily lead to a 40 dB higher path loss compared to FRLL. A 40 dB path loss for the D.sup.4 propagation model leads to 100 times more base stations (BS) than for the FRLL-concept if the transmitting power is the same (essentially due to ten times less worst case range). Furthermore, as mentioned above, in some countries the telephone operators are prevented from offering mobility through the particular regulations in said countries. Furthermore, due to the above-mentioned limitations, MRLL is up to now only available as test systems and has not yet been commercially implemented. This is further described in the paper "Universal Digital Portable Communications: an Applied Research Perspective", ICC '86, Toronto, Canada, Jun. 22-25, 1986 by D. C. Cox et al.
A schematic illustration of the MRLL-concept is shown in FIG. 3 where subscriber mobile stations, SMS, 5', (portable telephones) are directly communicating with a base station 1', 1" via a radio connection. As can be seen the connection is direct as well for indoor as for outdoor connections. The long dashed line indicates that there is a high level of interference between base stations.
If so wanted, mobility in a residence that has normal WLL or FRLL can of course be provided by a private purchase of a standard commercially available cordless telehphone. This is illustrated in FIG. 4 where a separate cordless fixed part (CFP) is connected to the subscriber socket 9. The CFP can communicate with the cordless portable part (CPP). The cordless telephone is however not a part of the basic local loop provision made by the operator but merely a privately allowed extension. Furthermore, two separate radio systems have to be tandemed which leads to an extra delay of the speech signal, as well as it is expensive. Furthermore, if the cordless phone uses digital transmission digital speech coding/decoding has to be carried out twice. The mobility further is limited and different frequency bands are required for the private and the public link respectively.